A PWM converter has been well known, which is made up of Graetz-connected transistors or self-extinguishing elements such as gate turn-off thyristors. The magnitude and phase of an input current can be controlled if the a-c input terminals of the PWM converter are connected to an a-c power supply via reactors, and if the magnitude and phase of an a-c input voltage are instantaneously controlled with the a-c power supply voltage as a reference. It is therefore possible to arbitrarily control the effective electric power which is supplied from the a-c power supply to the PWM converter. For instance, Japanese Patent Laid-Open No. 94583/1980 proposes an apparatus and a method for this purpose. Since the effective power is converted by the PWM converter into d-c power, it is possible to control the d-c output voltage (current) to any desired value.
Feedback diodes have been connected in parallel, but in the opposite direction, with the self-extinguishing elements which constitute the PWM converter. To control the PWM converter by pulse width modulation, the d-c output voltage must be greater than diode-rectified voltage of the a-c power supply voltage. This is because, if the d-c voltage becomes smaller than the diode-rectified voltage the a-c power supply voltage, the d-c power is supplied through the feedback diodes of the PWM converter to hinder the pulse width modulation control.
Further, when the load of the PWM converter is subjected to a regenerative operation, the PWM converter will also be subjected to the regenerative operation. In this case, the d-c voltage is likely to become considerably greater than the diode-rectified voltage.
Depending upon the load connected to the d-c side of the PWM converter, on the other hand, the d-c voltage must often be equal to, or smaller than, the diode-rectified voltage of the a-c power supply voltage. When the load is an inverter or a chopper, this is necessary to prevent the elements from breaking.
Thus, when the PWM converter is used, it is strongly recommended to control the d-c voltage to make it equal to the diode-rectified voltage of the a-c power supply voltage.